AI Summary
Researchers have discovered a branched biosynthetic pathway for the production of mycosporine-like amino acids (MAAs), which are microbial sunscreens. By isolating a cyanobacterium from lake sediment and analyzing its genome, they identified genes responsible for the production of MAAs. Through cloning and expression in E. coli, they were able to produce glycosylated MAAs with different amino acid substitutions. This research sheds light on the genetic and enzymatic basis for the chemical diversity of MAAs, offering potential applications in sunscreen development.
Mycosporine-like amino acids (MAAs) are a family of water-soluble and colorless secondary metabolites with high extinction coefficients that function as microbial sunscreens. MAAs share a cyclohexinimine chromophorethat is diversified through amino acid substitutions and attachment of sugar moieties. The genetic and enzymatic basis for the chemical diversity of MAAs remain largely unexplored. Here we report a series of structurally distinct MAAs and evidence for an unusual branched biosynthetic pathway from a cyanobacterium isolated from lake sediment. We used a combination of high-resolution liquid chromatography–mass spectrometry (HR-LCMS) analysis and nuclear magnetic resonance (NMR) spectroscopy to identify diglycosylated-palythine-Ser (C22H36N2O15) as the dominant chemical variant in a series of MAAs from Nostoc sp. UHCC 0302 that contained either Ser or Thr. We obtained a complete 9.9 Mb genome sequence to gain insights into the genetic basis for the biosynthesis of these structurally distinct MAAs. We identified MAA biosynthetic genes encoded at two locations on the circular chromosome. Surprisingly, direct pathway cloning and heterologous expression of the complete mysABC1JD1G1H biosynthetic gene cluster in Escherichia coli BL21(DE3) led to the production of 450 Da monoglycosylated-palythine-Thr (C18H30N2O11). We reconstructed combinations of the two distant biosynthetic gene clusters in refactored synthetic pathways and expressed them in the heterologous host. These results demonstrated that the MysD1 and MysD2 enzymes displayed a preference for Thr and Ser, respectively. Furthermore, one of the four glycosyltransferases identified, MysG1, was active in E. coli and catalysed the attachment of a hexose moiety to the palythine-Thr intermediate. Together these results provide the